1. Field of the Invention
The present invention relates to a liquid crystal display device in which a semiconductor device such as a transistor formed on an insulator, in particular, a field effect transistor, and typically a MOS (metal oxide semiconductor) transistor, a thin film transistor (hereinafter, referred to as TFT) is used as a switching element in a pixel portion. The present invention also relates to a liquid crystal display device including a circuit or a driving circuit manufactured by using the semiconductor device, and to an electric appliance in which the liquid crystal display device is used for a display portion.
2. Description of the Related Art
Recently, there has been growing a use of the liquid crystal display device in a monitor of a personal computer or a display device of a television, in which liquid crystal is interposed between a pair of substrates and an electric field is applied to the pair of substrates to perform a display through a liquid crystal orientation.
Further, due to an improvement in a technique of crystallizing a semiconductor film, the liquid crystal display device is also realized in which a driving circuit is incorporated into one substrate.
Here, for the TFT of the driving circuit, high field effect mobility is required, whereas low leak current characteristics are required for the TFT used as the switching element in each pixel of the pixel portion. Thus, it is important to hold a charge (signal) and even a slight amount of leak current generated during a holding time at which the TFT is in an OFF state causes a deterioration of an image quality and a decrease in a contrast.
However, the TFT involves such a problem that when a semiconductor layer is irradiated with a light, optical excitation occurs to generate an optical leakage current. Therefore, it is important that a light-shielding film for covering the TFT is formed to block the light sufficiently to thereby prevent the light irradiation to the semiconductor layer or, assuming that a leak current is generated, it is important to secure a storage capacitor large enough to hold a signal in one frame period even in such a case.
Thus, as a structure in which a storage capacitor is formed and further a leak light can be shielded, a cell is disclosed in JP 2924506 B, in which a source electrode and a drain electrode are formed before an interlayer film is formed and a light-shielding film made of aluminum is formed on the interlayer film and subjected to anodic oxidation to form an anodized film made of Al2O3 on a top surface and a side surface of the light-shielding film, and then a transparent pixel electrode is formed thereon to complete the cell including the storage capacitor consisting of the light-shielding film, an Al2O3 film, and the transparent pixel electrode that are arranged in the stated order.
An electrostatic capacitance (hereinafter, referred to as a capacitance) of a storage capacitor element is in inverse proportion to a thickness of a capacitor insulating film (in this specification, referred to as a dielectric film sandwiched between a pair of electrodes constituted of a conductor) and in proportion to a dielectric constant of the capacitor insulating film and a surface area of the electrode. Therefore, in the structure disclosed in JP 2924506 B, since the capacitor insulating film between the light-shielding film and the transparent pixel electrode is made of the anodized film and maintained almost constant, a region where the light-shielding film and the transparent pixel electrode overlap each other can substantially function as the storage capacitor element.
However, in the liquid crystal display device, there arises a problem in that a defective display is caused by a defect in orientation of the liquid crystal. The defect in orientation of the liquid crystal occurs as follows. Steps or unevennesses on the surface of the pixel electrode affect the surface of an orientation film and involve uneven rubbing on the orientation film, and finally causes the defect in orientation of the liquid crystal and the deterioration of the image quality. In the structure disclosed in JP 2924506 B, as shown in FIG. 1A, the step of the pixel electrode exists in a portion outside a light-shielded region by the light-shielding film (a region transmissive of light) and there arises a problem in that the light accidentally passes therethrough due to the defect in orientation of the liquid crystal, or the like.
However, in the display device including a laminate structure consisting of a number of layers, it is unavoidable that the steps or unevennesses occur on the surface of the pixel electrode and affect the surface of the orientation film as well, followed by decrease in the contrast due to a light accidentally passing therethrough. Thus, a method of flattening the steps or unevennesses of the pixel electrode, particularly, a region transmissive of light which contributes to a display of the pixel has been considered.
Thus, a method is devised in which the light-shielding film is formed on an element substrate side and the step caused through the formation is flattened to form the pixel electrode. However, according to this method, when the storage capacitor element is formed between the light-shielding film and the pixel electrode, as shown in FIG. 1B, an interlayer distance is made large between the light-shielding film and the pixel electrode to thereby make a region functioning as the capacitor element narrow, with the result that it is impossible to secure the sufficient storage capacitor.
Due to a slight amount of off-leak current flowing while the TFT is in an OFF state, the decrease in the contrast and a nonuniform image quality as a panel are caused, so that it is unavoidable that the storage capacitor element is formed for complementation thereof. However, if the storage capacitor element is formed in another region, for example, as shown in FIG. 1C, if the storage capacitor element is constituted of a semiconductor layer, a gate insulating film, and a conductive layer formed on the gate insulating film with an active layer of the TFT extended, an opening ratio of the pixel becomes small, so that a display capacity is decreased in terms of brightness.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to realize a semiconductor device including a capacitor element capable of obtaining a sufficient storage capacitor without reducing an opening ratio, in which a pixel electrode is flattened in order to control a defect in orientation of liquid crystal.
According to the present invention, there is provided a semiconductor device, including: a thin film transistor; a storage capacitor element; a light-shielding film formed on the thin film transistor; a capacitor insulating film formed on the light-shielding film; a conductive layer formed on the capacitor insulating film; an insulating film formed on the conductive layer; and a pixel electrode formed on the insulating film, in which: the conductive layer and the pixel electrode are electrically connected; and the storage capacitor element is composed of the light-shielding film, the capacitor insulating film, and the conductive layer.
Also, according to the present invention, the semiconductor device includes: a light-shielding film formed on a thin film transistor; a capacitor insulating film formed on the light-shielding film; a conductive layer formed on the capacitor insulating film; an insulating film formed on the conductive layer; and a pixel electrode formed on the insulating film, in which: the storage capacitor element is composed of the light-shielding film, the capacitor insulating film, and the conductive layer; and the conductive layer and the pixel electrode are brought into contact with each other through an opening formed in the insulating film, the opening formed on the light-shielding film.
Further, according to the present invention, there is provided the semiconductor device, including: a thin film transistor; a storage capacitor element; a light-shielding film formed on the thin film transistor; a capacitor insulating film formed on the light-shielding film; a conductive layer formed on the capacitor insulating film; an insulating film formed on the conductive layer; and a pixel electrode formed on the insulating film, in which: the storage capacitor element is composed of the light-shielding film, the capacitor insulating film, and the conductive layer; the conductive layer and the pixel electrode are brought into contact with each other through an opening formed in the insulating film; the insulating film is formed so that a step of the pixel electrode caused by the capacitor insulating film and the conductive layer is flattened; and an inner wall of the opening and the step are formed on the light-shielding film.
Further, according to the present invention, there is provided the semiconductor device, including: a thin film transistor; a storage capacitor element; a light-shielding film formed on the thin film transistor; a capacitor insulating film formed on the light-shielding film; a conductive layer formed on the capacitor insulating film; an insulating film formed on the conductive layer; and a pixel electrode formed on the insulating film, in which: the storage capacitor element is composed of the light-shielding film, the capacitor insulating film, and the conductive layer; the conductive layer and the pixel electrode are brought into contact with each other through an opening formed in the insulating film; and the insulating film is formed so that a step between one pixel electrode and other pixel electrode adjacent thereto is flattened.
Further, according to the present invention, there is provided the semiconductor device, including: a thin film transistor; a storage capacitor element; a light-shielding film formed on the thin film transistor; a capacitor insulating film formed on the light-shielding film; a conductive layer formed on the capacitor insulating film; an insulating film formed on the conductive layer; and a pixel electrode formed on the insulating film, in which: the storage capacitor element is composed of the light-shielding film, the capacitor insulating film, and the conductive layer; the conductive layer and the pixel electrode are brought into contact with each other through an opening formed in the insulating film; and the conductive layer and the pixel electrode are aligned at end surfaces thereof between adjacent pixels.
Further, according to the present invention, there is provided the semiconductor device, including: a thin film transistor; a storage capacitor element; a light-shielding film formed on the thin film transistor; a capacitor insulating film formed on the light-shielding film; a conductive layer formed on the capacitor insulating film; an insulating film formed on the conductive layer; and a pixel electrode formed on the insulating film, in which: the storage capacitor element is composed of the light-shielding film, the capacitor insulating film, and the conductive layer; the conductive layer and the pixel electrode contact through an opening formed in the insulating film; and the capacitor insulating film, the conductive layer, and the pixel electrode are separately formed in each of pixels and are aligned at end surfaces thereof between the adjacent pixels.
The conductive layer formed on the capacitor insulating film is electrically connected to the pixel electrode so as to make a potential equal therebetween and the storage capacitor element composed of the light-shielding film, the capacitor insulating film formed on the light-shielding film, and the conductive layer formed on the capacitor insulating film is adapted thereto, so that a sufficient storage capacitor can be obtained without reducing an opening ratio.
Also, on the conductive layer, a flattening film formed of the insulating film is formed in order to prevent the defect in orientation of the liquid crystal due to the steps or unevennesses on the surface of the pixel electrode in a region through which a light is passed (region contributing to a display). Through the formation of the flattening film, since the step of the pixel electrode is formed on the light-shielding film, even if an uneven rubbing treatment is performed on the orientation film to cause the defect in orientation, it does not affect a display.
Note that, according to the present invention, the liquid crystal is sandwiched between a pair of substrates and the thin film transistor is used as a switching element for each pixel. The present invention can be adapted to all liquid crystal display devices including a liquid crystal display device connected to a driving circuit through a connector such as an FPC or a TAB, a liquid crystal display device in which a pixel portion and a driving circuit are integrally formed on the same substrate (hereinafter, referred to as integrally formed liquid crystal display device), a liquid crystal display device in which the driving circuit is formed in the integrally formed liquid crystal display device and the pixel portion is connected with a controller having a function of displaying an image, and a liquid crystal display device provided with a microcomputer for controlling the controller. Further, in this specification, all the liquid crystal display devices described above collectively refer to the semiconductor device.